This invention relates generally to pulsed coherent lasers, and more particularly the invention relates to the reduction of speckle in coherent laser pulses such as used in microlithography in the fabrication of semiconductor integrated circuits.
Microlithography is used in the definition of circuit components in the fabrication of integrated circuits. With the advent of submicron line widths in circuit definition, both x-ray beams and laser beams have been employed for photoresist development. Eximer lasers have multiple spatial mode characteristics and high average power which is well suited for use in microlithography.
Coherent lasers possess many advantages over eximer lasers in microlithography due to the shorter wavelength and narrower spectral width. This leads to higher efficiency and smaller size. However, in coherent laser sources the "speckle" or random intensity distribution of light due to interference of highly coherent laser beams adversely affects the development of photoresist patterns in microlithography. In the ultraviolet regions, all presently available alternatives to eximer lasers are highly coherent pulsed laser systems, and currently proposed speckle reduction schemes work only for short coherent length laser systems or for wavelengths above the ultraviolet region.
The present invention is directed to an apparatus and method for reducing speckle in coherent laser pulses.